1. Field of the Invention
The present invention relates to a phase-shifting mask, and in particular to a structure and method of correcting proximity effects in a tri-tone attenuated phase-shifting mask.
2. Description of the Related Art
Photolithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional photolithography, a mask (or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. The emanating light from the mask is then focused on a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of the mask is transferred to or printed on the photoresist layer.
However, diffraction effects at the transition of the transparent regions to the opaque regions can render these edges indistinct, thereby adversely affecting the resolution of the photolithography process. Various techniques have been proposed to improve the resolution. One such technique, phase-shifting, uses phase destructive interference of the waves of incident light. Specifically, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a more sharply defined interface between the first and second regions. Thus, the boundary between exposed and unexposed portions of a photoresist illuminated through a semiconductor mask (or reticle) can be more closely defined by using phase-shifting, thereby allowing greater structure density on the IC.
FIG. 1 illustrates a simplified, phase-shifting mask 100 fabricated with an attenuated, phase-shifting region 101 formed on a clear region 102, wherein a border 103 of attenuated, phase-shifting region 101 defines a single IC structure. Clear region 102 is transparent, i.e. a region having an optical intensity transmission coefficient T>0.9. In contrast, attenuated phase-shifting region 101 is a partially transparent region, i.e. a region having a low optical intensity transmission coefficient 0.03<T<0.1. The phase shift of light passing through attenuated phase-shifting region 101 relative to light passing through clear region 102 is approximately 180 degrees.
As known by those skilled in the art, increasing the intensity transmission coefficient of attenuated phase-shifting region 101 could increase the performance of structures formed by the photolithographic process. In fact, optimal performance would be theoretically achieved by providing an attenuated, phase-shifting region with an optical intensity transmission coefficient approaching T=1 (in other words, the region would be transparent) yet having a phase shift of 180 degrees relative to clear region 102. In this manner, assuming partially coherent illumination, amplitude side lobes from each region would substantially cancel, thereby creating a substantially zero-intensity line at the transition between these two regions. Current material technology typically provides this phase shift with an attenuated, phase-shifting region having an optical intensity transmission coefficient of approximately T=0.4, although a higher transmission is theoretically possible and preferable.
Unfortunately, the use of this higher transmission phase-shifting material increases the risk of printing certain portions of attenuated phase-shifting region 101. Specifically, to ensure complete removal of residual photoresist, the actual dose used to remove the photoresist is typically at least twice the theoretical dose needed to remove the photoresist. This over-exposure can result in increasing the risk of printing certain larger portions of attenuated phase-shifting region 101.
To solve this problem, some masks, called tri-tone attenuated phase-shifting masks, include an opaque region within the larger portion(s) of the attenuated, phase-shifting region, wherein the opaque region blocks any unwanted light transmitted by the attenuated phase-shifting region. FIG. 2 illustrates a simplified, phase-shifting mask 200 fabricated with an attenuated phase-shifting region 201 formed on a clear region 202 and an opaque region 204 formed on attenuated phase-shifting region 201, wherein a border 203 of attenuated phase-shifting region 201 defines a single IC structure. In this embodiment, clear region 202 has an optical intensity transmission coefficient T>0.9, attenuated phase-shifting region 201 has an optical intensity transmission coefficient 0.3<T<0.9, and an opaque region 204 typically has an intensity transmission coefficient of T<0.01. Note that the phase shift of light passing through attenuated phase-shifting region 201 relative to light passing through clear region 202 remains approximately 180 degrees.
Thus, forming an opaque region on an attenuated phase-shifting region advantageously allows for the use of a significantly higher optical intensity transmission coefficient for isolated structures. Unfortunately, a tri-tone phase-shifting mask exhibits strong optical proximity effects, thereby making it difficult to utilize this mask in a single common exposure for both isolated as well as crowded patterns.
Therefore, a need arises for a structure and a method for correcting optical proximity effects on a tri-tone, attenuated phase-shifting mask.